A technology for preventing leakage of a fuel component to the outside of the vehicle by introducing fuel gas volatilizing in a fuel tank of a vehicle into a cylinder of an engine is known. Fuel gas in the fuel tank is temporarily absorbed by a canister, and fuel gas desorbed from the canister (this is called “purge gas”) is introduced into an intake passage. On a purge passage that connects the canister and the intake passage to each other, a purge controlling valve for adjusting the flow rate of the purge gas is disposed, and the opening of the purge controlling value is controlled in response to the operating condition of the engine.
Incidentally, the air-fuel ratio of air-fuel mixture introduced into a cylinder of an engine during introduction of purge gas varies in response to the concentration of the purge gas. Therefore, a technology for controlling the air-fuel ratio appropriately by estimating the concentration of the purge gas with a high degree of accuracy has been developed. For example, a technology of providing an air-fuel ratio sensor on an exhaust passage to detect an air-fuel ratio and estimating the concentration of purge gas based on a difference between the detected air-fuel ratio and a target air-fuel ratio is known. Also a technology of calculating an air-fuel ratio feedback correction coefficient that corresponds to a ratio between an air-fuel ratio and a target air-fuel ratio and learning the concentration of purge gas based on a variation of the correction coefficient is available (for example, Japanese Laid-Open Patent Publication No. Hei 7-63078 (JPA 1995-063078)).
However, according to a concentration calculation technique based on an air-fuel ratio, the calculation error tends to increase as the flow rate of purge gas decreases.
A relationship between an air-fuel ratio detected by an air-fuel ratio sensor on an exhaust passage and a concentration and a purge gas flow rate of purge gas is exemplified as a graph in FIG. 7. This graph particularly indicates a relationship among three factors including the concentration of purge gas, the purge gas flow rate and the air-fuel ratio detected by a sensor when air-fuel mixture of purge gas of an arbitrary air-fuel ratio and fresh air of a stoichimetric air-fuel ratio is supplied into a cylinder. If this relationship is used, then it is possible to estimate the concentration of purge gas from a purge gas flow rate and an air-fuel ratio.
When the value of the air-fuel ratio detected by an air-fuel ratio sensor is equal to a stoichimetric air-fuel ratio, it is estimated that the concentration of the purge gas exhibits the stoichimetric air-fuel ratio irrespective of the magnitude of the flow rate of the purge gas. On the other hand, when the value of the air-fuel ratio detected by the air-fuel ratio sensor is lower (richer) than the stoichimetric air-fuel ratio, the estimated value of the concentration of the purge gas increases as the flow rate of the purge gas decreases. On the contrary, when the value of the air-fuel ratio is higher (leaner) than the stoichimetric air-fuel ratio, the estimated value of the concentration of the purge gas decreases as the flow rate of the purge gas decreases.
As described above, as the flow rate of purge gas decreases, the estimated value of the concentration of the purge gas fluctuates by an increasing amount with respect to a small variation of the value of the air-fuel ratio. Accordingly, in an operating condition in which the opening of the purge controlling valve is controlled to a comparatively low value, the estimation accuracy of the concentration of purge gas is apt to degrade, and the controllability of the engine may degrade.
It is to be noted that, if the calculation accuracy of the air-fuel ratio can be enhanced, then also the estimation accuracy of the concentration of purge gas enhances. However, it is difficult to prevent occurrence of a detection error by a dispersion of the detection accuracy caused by an individual difference of an air-fuel ratio sensor or by a time-dependent degradation. Therefore, there is a situation that, for a control apparatus for an engine incorporated in a vehicle on the market, a controlling technique for implementing concentration calculation of purge gas that is not influenced by the calculation accuracy of the air-fuel ratio is sought.